I picked up a trick somewhile back when trying to figure out why my machine no matter how many parts I replaced would not power on, I changed the PSU first, Then the ram, then the MB and still no fans no beeps (post) so I took out everything but the PSU and MB, guess what.. I got fans and beeps (post) so I put everything back in 1 piece at a time, starting with ram and then I got to the GPU and boom no fan no post. so it was just a bad GPU causing the whole machine to not work.. had the green light on the board but nothing else.

So if your PC has no fans or no beep (post) when you press the power button.

Pull out EVERYTHING but the PSU and Motherboard, and if it wont beep then.. try a new PSU and then MB, or if it beeps start putting things back 1 piece at a time, booting each time until you find the defective hardware.

Thats why when i upgrade several items at once or build i test run after certain items.
I installed a TX750 hooked it all up ran the system, prior to installing the GTX260
Same goes for upgrades for me as it does trouble shooting.. if its multiple items its good to go one at a time that way if you run into trouble its easy to narrow down.
I also normally post on the table with the mobo out of the case on a new build but i do it
CPU, PSU, RAM (given theres onboard graphics ofcoarse) and short the switch make sure i have post before i install it in the case. just habit.

When mine didn't post I did something similar however I left all the internal parts in and removed my usb hub, ipod, mouse etc and that helped. But as said earlier it is a standard proceudure to booth with only barebone components.